Tensile Bond Strength and Retention of Three Types of Ceramic Endocrowns

Statement of the Problem: By development of adhesive dentistry and noble mechanical strength of ceramics, reconstruction of posterior teeth with partial coverage restorations such as ceramic endocrowns is possible. Different ceramics may show different mechanical properties which should be investigated. Purpose: The aim of this experimental in vitro study was to compare the tensile bond strength of endocrowns made by CAD-CAM using 3 types of ceramics. Materials and Method: In this in vitro study, 30 fresh extracted human molars were prepared to evaluate the tensile bond strength of endocrowns made by IPS e.max CAD, Vita Suprinity, and Vita Enamic blocks (n=10). The specimens were mounted and endodontically treated. Standard preparations were done with 4.5±0.5 mm intracoronal extensions into the pulp chamber and the restorations were designed and milled by CAD-CAM technique. All specimens were cemented with a dual polymerizing resin cement according to the manufacturer's instructions. The specimens were incubated for 24 hours and then thermocycled for 5000 cycles at 5-55°C and underwent the tensile strength test by universal testing machine (UTM). Shapiro-Wilk and one-way ANOVA test were used to statistically analyzed (α= 0.05). Results: The highest tensile bond strength values were achieved in IPS e.max CAD (216.39 ±22.67N) and Vita Enamic (216.22±17.72N) followed by Vita Suprinity (211.54±20.01N). There was no significant statistical difference between retention of endocrowns made by CAD-CAM technique among ceramic blocks (p= 0.832). Conclusion: Within the limitation of this study, there was no significant difference between retention of endocrowns made by IPS e.max CAD, Vita Enamic, and Vita Suprinity as ceramic blocks.


Introduction
Reconstruction of extensively damaged teeth is a common challenging issue in restorative dentistry [1]. Many factors such as bond strength of an adhesive system, the restorative material thickness, proximity of the modulus of elasticity between the restoration material and tooth structure, and the presence of microleakage affect the longevity of the restoration [1][2].
There are different methods to restore endodontically treated teeth. Direct build-up restorations traditionally include prefabricated post and amalgam core, which can lead to unsatisfactory outcomes like root fracture, microleakage, and material deterioration. Following the development of adhesive systems and glass ionomer cement, the microleakage problem was resolved to a great extent, leaving other problems unchanged [3].
Usage of cast post and core is another way to stabilize the restoration of endodontically treated teeth [4].
Studies showed that this treatment modality failure could be due to the mechanical characteristics of tooth structure [4]. Disadvantages like re-infection of the root canal system, tooth perforation, dependency on root morphology, increased chair-time, and prolonged laboratory procedures, give rise to the prevalent application of adhesive restorations with the pulp chamber as the main element of retention [3].
The concept of endocrowns was first introduced by Pissis in 1995. Subsequently, Bindl and Mormann have extended this term in 1999. These materials can rehabilitate teeth without adequate ferrule effect by utilizing the macro-retentive of pulp chamber walls and microretentive of adhesive cementations. In the meantime, omitting the need for post and core restorations put the endocrowns among conservative treatment options [5].
Over the past 10 years, computer-aided design/ computer-aided manufacturing (CAD-CAM) technology had extensive advertising that can be used instead of conventional lab techniques [6]. This method improves the mechanical properties of materials and also eliminates defects of manual procedures [7].
Dental ceramics are the main categories of materials that can be used with CAD-CAM technology [6]. These materials are subdivided into three principal subgroups: glass-matrix ceramics, polycrystalline ceramics, and resin-matrix ceramics [6]. The first two groups include inorganic ceramic materials. The resin-matrix ceramic is made of polymer that contains inorganic compositions such as glass, ceramics, and glass-ceramics [8].  [9].
The first zirconia-reinforced lithium silicate ceramics like Vita Suprinity (Vita Zahnfabrik, Bad Sackingen, Germany) has been introduced for dental CAD-CAM which aims to mix the favorable material characteristics of both lithium silicate ceramic and zirconia (10% by weight). This synthetic material is a subcategory of the glass-matrix ceramics in which adding zirconia result in a round and slightly elongated crystalline structure with an average size of 0.5µm. This phase reinforces the ceramic structure by limiting the crack propagation [6,8,10].
To improve mechanical proportions, a new network material in which a porous ceramic is infiltrated by a polymer has been revealed by the Vita Company. The main trait of this resin-matrix ceramic is a fine-grain crystal in the structure without a glassy phase. Vita Enamic (Vita Zahnfabrik, Bad Sackingen, Germany) is a polymer-infiltrated ceramic network (PICN) [11]. The construction procedure provides a suitable brittleness index for CAD milling unite [6]. Furthermore, the high modulus elastic, and hardness of this material illustrates similar creep behavior to human enamel, which provides sufficient durability of tooth restorations [10].
Retention of endocrowns is an important factor in the longevity of these restorations. Therefore, this study aimed to evaluate the retention of endocrowns made by CAD-CAM using three types of ceramics including IPS e.max CAD, Vita Suprinity, and Vita Enamic. The null hypothesis of the study was defined, as there is no difference between the tensile bond strength of three tested ceramics.

Preparation of tooth specimen
Thirty extracted fresh mandibular molars with completed roots and without cracks, fractures, or decays were cllected and stored in saline solution for 7 days ( Figure 1).
To standardize the size of the selected teeth, a digital caiper (S235, Sylvac, Switzerland) was used to measure the buccolingual and mesiodistal dimensions of each tooth at the cement-enamel junction (CEJ) Thinner or thicker teeth from 8 to 10mm were excluded from the study [2,12]. Each tooth was mounted in a prefabricated aluminum mold (25×25mm) with the occlusal surface parallel to the horizontal plane, using self-curing acrylic resin   (1): IPS e.max CAD endocrowns, Group (2): Vita Suprinity endocrowns, and Group (3): Vita Enamic endocrowns. All materials and appliances are described in detail in Table 1.

Endocrown preparation
Special milling machine (Centroid CNC, milling machine, USA) was used to standardize the preparation of the specimens. The teeth were cut 3-mm above the CEJ.  (Figure 2).
The appropriate design software (2017, 3Shape Dental System) was used to design endocrowns on the virtual All endocrowns were polished using their special polishing kit without additional glazing.

Bonding procedure
Etching with 5% hydrofluoric acid (IPS Ceramic Etch

Thermal cycling
The samples were placed in a thermocycling device (T- The data were analyzed by Shapiro-Wilk and one-way ANOVA tests. The significance level was 0.05%.

Results
The mean and SD values of tensile bond strength and retention for all groups are illustrated in Table 2 and   As the distribution of the data proved to be normal by Shapiro-Wilk Test, parametric tests were selected for comparing the data. Based on the results of the one-way analysis of variance in Table 3, there was no significant difference between tensile bond strength of endocrowns made by CAD-CAM technique regarding the ceramic type (p= 0.832).

Discussion
There is no consensus on the best treatment plan for posterior endodontically treated teeth; residual coronal structure seems to be the most important factor in the long-term prognosis of the restoration [5]. Although the use of post-core-crown has become a classic method for the reconstruction of severely damaged teeth, this notion has changed since the advent of adhesives in conservative dental treatment [5].
Resistance to masticatory forces and proper retention is one of the most effective items in the clinical prognosis of conservative restoration [12]. In the oral environment, restoration is affected by various forces such as compressive, tensile, and shear forces intermittently and frequently. Accumulation of these forces may lead to a failure in the bonding interference of tooth and the restoration in the long term, leading to the loss of retention GPa) [14][15].
Brittle ceramic blocks may initiate crack at adhesive /ceramic interface at lower values than the more resilient ones like Enamic. In addition, after design and milling, Enamic blocks do not need to be placed in a furnace to complete crystallization, which reduces the chair time [16]. In this study, the average tensile bond strength of endocrowns made by Enamic was higher than Suprinity, but this difference was not statistically significant.
Zirconia polycrystalline structure is the main reason for the weak bond strength of Y-TZP ceramics [17].   [20][21]. A total of 30-60 seconds etching is also recommended for polymer-infiltrated ceramics for maximum tensile bond strength [22][23]. In the present study, E.
max CAD and Suprinity were etched for 20 seconds and Enamic for 60 seconds. In addition, ceramic etching leads to exposure of hydroxyl groups and allows chemical interactions with the Silane coupling agent [24][25].
The use of a silane coupling agent to increase the surface energy and improve retention between resin cement and restorative material has been widely suggested [26].
Therefore, in this study, silane coupling agent was used for all specimens.
Multi-stage application of total-etch adhesives, and their increased chair time serve as main disadvantages in the bonding procedure. Because of these limitations, self-adhesive resin cements were introduced for decreasing the whole process and shortening the window of contamination [27][28]. Therefore, self-adhesive resin cements are recommended for bonding endocrown restorations according to similar articles and were chosen as a luting agent in the present study [29].
Jing et al. [30] showed that increasing the occlusocervical height of the preparation leads to an increase in the tensile bond strength of the restoration. In the present study, the mean of pulp chamber height was considered to be 4.5±0.5mm. There have been some limitations in this study. Differences in pulp chamber morphology could make bias in results. Further studies are needed to investigate the mechanical and adhesive properties of the materials used in and even long-term follow-up sessions in the clinic.

Conclusion
The present study showed that the use of IPS e.max CAD, Suprinity, and Enamic ceramic blocks to build indirect conservative restorations is promising. However, further studies are needed to investigate the mechanical and adhesive properties of endocrowns in prolonged follow-up sessions.